The present invention relates to a single layer high performance three-way catalyst (TWC) containing a catalytic coating comprising platinum, rhodium and various oxide materials on an inert carrier body.
Three-way catalysts are used to convert the pollutants carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in the exhaust gas of internal combustion engines into harmless substances. Known three-way catalysts with good activity and durability utilize one or more catalytic components from the platinum group metals such as platinum, palladium, rhodium deposited on a high surface area, refractory oxide support, e.g., a high surface area alumina. The support is usually carried in the form of a thin layer or coating on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure.
The ever increasing demand for improved catalyst activity and life has led to complex catalyst designs comprising multiple catalyst layers on carrier structures, each of the layers containing selected support materials and catalytic components as well as so called promoters, stabilizers and oxygen storage compounds.
For applying the different layers onto the carrier structures so-called coating dispersions, coating compositions or washcoat compositions are prepared which comprise the support materials in finely divided form and optionally additional soluble components. The liquid phase of the coating composition is preferably water. This coating composition is used to apply the catalytic coating onto the carrier structures. The techniques for applying the coating are well known to the expert. The fresh coating is then dried and calcined to fix the coating and to transform the optional soluble components of the coating composition into their final insoluble form.
For the production of double or multiple layer catalysts a dedicated coating composition for each layer has to be provided. This increases the production costs. Therefore, it is one object of the present invention to design a single layer catalyst which approximates the catalytic properties of sophisticated multiple layer catalysts.
Modern three-way catalysts make use of the platinum group metals platinum, palladium and rhodium. Platinum and palladium mainly promote the oxidation of hydrocarbons (HC) and carbon monoxide (CO) and may be present in the catalyst simultaneously or alternatively. Rhodium primarily promotes the reduction of nitrogen oxides (NOx). While platinum and palladium may replace each other to a certain extent, this is not the case for rhodium. The exhaust gas cleaning efficiencies promulgated by the most recent legal exhaust gas standards can only be met at reasonable cost by using rhodium together with one or both of platinum and palladium.
On the other hand it was observed that rhodium containing three-way catalysts suffer under the so-called fuel-cut ageing. The term fuel-cut ageing describes catalyst performance degradation due to fuel-cut after high load operation of the internal combustion engine. Such a situation occurs frequently during fast driving phases when abrupt deceleration is required. During fast driving phases the engine is operated at air/fuel ratios slightly below the stoichiometric value. The exhaust gases may reach temperatures well above 900xc2x0 C. resulting in even higher catalyst temperatures due to the exothermic conversion reactions at the catalyst. In case of abrupt deceleration modern motor electronics completely stop fuel supply to the engine with the result that the normalized air/fuel ratio (also called lambda value xcex) of the exhaust gas jumps from rich to lean values.
These large excursions of the normalized air/fuel ratio from rich to lean values at high catalyst temperatures degrade catalytic activity. Catalytic activity can at least partly be recovered by prolonged operation under stoichiometric or rich exhaust gas conditions. The faster catalytic activity is regained after fuel-cut ageing the better is the overall catalyst performance. Speeding up recovery of catalytic activity after fuel-cut ageing is therefore mandatory for modern three-way catalysts.
Therefore, it is another object of the present invention to provide a catalyst with higher resistance towards fuel-cut ageing. That is, after high temperature ageing under lean exhaust gas conditions, the catalyst should recover its full three-way efficiency quickly. Reduced fuel-cut ageing will also improve the overall dynamic behaviour of the catalyst.
U.S. Pat. No. 4,965,243 discloses a single layer three-way catalyst comprising, on activated alumina, platinum and rhodium in a weight ratio of 5:1 and further cerium oxide, barium oxide and zirconium oxide. This combination of components is said to be very effective for maintaining an excellent catalyst activity even after the catalyst has been exposed to high temperatures of 900 to 1100xc2x0 C.
U.S. Pat. No. 5,200,384 describes a single layer three-way catalyst comprising, on activated alumina, platinum and rhodium in a weight ratio of 5:1 and further cerium oxide an a coprecipitated ceria-stabilized zirconia having a weight ratio of ceria to zirconia between 1:99 and 25:75. The addition of the coprecipitated ceria-stabilized zirconia to the three-way catalyst is said to enhance the activity of the catalyst at low temperature after high temperature ageing.
U.S. Pat. No. 5,254,519 discloses a single layer catalyst comprising a combination of a coformed rare earth oxide-zirconia having a rhodium component dispersed thereon and a first activated alumina having a platinum component dispersed thereon. The catalyst may comprise a second rhodium component dispersed on the first alumina support. Alternatively, the second rhodium component may be dispersed on a second alumina component.
During the last years there could be observed a tendency of replacing platinum in three-way catalysts completely with palladium because of its lower price and good oxidation activity. Palladium/rhodium and platinum/palladium/rhodium three-way catalysts had been developed which exhibited excellent catalytic activities at high palladium loads. Meanwhile the high demand for palladium has created a world-wide palladium shortage associated with a large increase of palladium prices. Nowadays palladium is more expensive than platinum. Therefore it is still another object of the present invention to provide a catalyst using platinum and rhodium with less precious metal costs but equivalent catalytic activity compared to palladium and rhodium containing catalysts.
These and further objects of the invention can be achieved with a single layer high performance catalyst containing on an inert carrier body a catalytic coating comprising platinum, rhodium and various oxide materials.
The catalyst is characterized in that the catalytic coating comprises
a) at least one first support material selected from the group consisting of a first active alumina, ceria rich ceria/zirconia mixed oxide and a zirconia component, said at least one first support material being catalyzed with a first part of the total platinum amount of the catalyst, and
b) a second support material catalyzed with the second part of the total platinum amount and with rhodium said second support material being a second active alumina.
The term xe2x80x9ca material is catalyzed withxe2x80x9d means that said material holds on its surface catalytically active components in highly dispersed form, such as platinum, rhodium or palladium.
The present invention is based on a co-pending European patent application of the inventors with publication number EP 1 046 423 A2. This application discloses a double layer catalyst with an inner and an outer layer on an inert carrier body comprising noble metals from the platinum group deposited on support materials. In the inner layer platinum is deposited on a first support and on a first oxygen storage component and in the outer layer platinum and rhodium are deposited on a second support and the second layer further comprises a second oxygen storage component.
The catalyst of the co-pending European patent application exhibits excellent catalytic properties compared to state of the art palladium and rhodium containing three-way catalysts. The present invention tries to reach similar catalytic properties with a single layer catalyst design to cut down production costs.
With the catalyst of the present invention, reduced fuel-cut ageing and improved dynamic behaviour and catalytic activities are obtained by placement of platinum and rhodium on dedicated support materials. The superior catalytic activity of the catalyst allows to reduce the precious metal loading while still maintaining catalytic activity comparable to state of the art three-way palladium/rhodium catalysts. This leads to reduced precious metal costs compared to conventional catalysts.
It is an essential feature of the present invention that all of the rhodium present in the catalyst is closely associated with platinum. This is accomplished by depositing the second part of the total platinum amount and rhodium onto the same particulate support material, the second active alumina.
According to the present understanding of the invention the reason for the reduced sensitivity against fuel-cut ageing may be that large excursions of the normalized air/fuel ratio from rich to lean values at high catalyst temperatures degrades the catalytic activity especially of rhodium. Under stoichiometric or rich exhaust gas conditions rhodium is reduced nearly to the oxidation state zero which is the most effective state for three-way catalysis. Under lean exhaust gases and at high catalyst temperatures rhodium gets oxidized up to oxidation level +3. This oxidation state of rhodium is less active for three-way conversion of pollutants. Moreover, since Rh2O3 is isomorphic in crystallographic structure to Al2O3 it can migrate at temperatures above 600xc2x0 C. into the lattice of alumina or other isomorphic support oxides of the general composition M2O3 (M stands for a metal atom), resulting in a permanent degradation of catalytic activity.
To regain its catalytic activity and to avoid losses of rhodium into the lattice of alumina rhodium must therefore be reduced as quickly as possible when the exhaust gas composition changes back to stoichiometry. According to the present understanding of the invention reduction of rhodium to oxidation state zero is catalyzed by platinum. The more intimate the contact between platinum and rhodium the better is this reduction effect.
In addition, the tendency of Rh2O3 to migrate into isomorphic support oxides can be limited by appropriate doping of these oxides. Beneficial are doping components which are capable of generating activated hydrogen under reducing conditions. The activated hydrogen helps to convert rhodium oxide more rapidly into the metallic form under reducing conditions and hence the risk of Rh2O3 migrating into the support oxide is further minimized. A suitable doping component for that purpose is cerium oxide (ceria). But since ceria also exhibits an oxygen storage and release capability the amount of doping with ceria must be kept low so as to not promote oxidation of rhodium by a too high level of ceria in the support oxide.
Further improvement of the ageing stability of the catalyst is achieved by proper selection of an oxygen storage component. Ceria is well-known to exhibit an oxygen storage capability. Under lean exhaust gas conditions cerium is completely oxidized to the oxidation state Ce4+. Under rich exhaust gas conditions ceria releases oxygen and acquires the Ce3+ oxidation state. Instead of using pure ceria as an oxygen storage compound the present invention uses ceria rich ceria/zirconia mixed oxide compounds. The term ceria rich denotes a material containing more than 50 wt.-% of ceria. Ceria concentrations of from 60 to 90 wt.-% relative to the total weight of the mixed oxide are preferred. Such materials are available with specific surface areas of 20 to 200 m2/g and exhibit a good temperature stability of the surface area. These materials are known to have a cubic crystalline habit of the type CeO2 as disclosed in U.S. Pat. No. 5,712,218. Further improvements can be obtained by stabilizing this material with praseodymia, yttria, neodymia, lanthana, gadolinium oxide or mixtures thereof. Stabilizing of oxygen storage materials based on ceria using praseodymia, neodymia, lanthana or mixtures thereof is described in German patent application DE 197 14 707 A1. Stabilization of ceria/zirconia mixed oxide with praseodymia is much preferred.
As already explained, the second part of the total platinum amount of the catalyst is in close contact to rhodium. This helps to reduce rhodium oxide formed during fuel-cut-off phases back to low oxidation state. For performing this task mass ratios between platinum and rhodium of 1:1 are most effective. Nevertheless, deviations from the 1:1 ratio between 3:1 and 1:5 have proven to still give good catalytic activities. While this mass ratio is valid for platinum and rhodium deposited together on the second active alumina the overall platinum/rhodium mass ratio in the catalyst may vary between 10:1 and 1:5, preferably between 10:1 and 1:1 with 3:1 being most preferred.
The zirconia component of the first support materials may be zirconia, optionally stabilized with 0.5 to 10 wt.-% of yttria, ceria, neodymia, lanthana, praseodymia, gadolinium oxide or mixtures thereof. Alternatively, the zirconia component may be equipped with an oxygen storage function by adding ceria in an amount sufficient to provide a substantial proportion of the total oxygen storage capacity of the catalyst. The ceria content of this zirconia component may vary from above 1 to below 50 wt.-% relative to the total weight of the zirconia component. Such materials are commercially available as so-called zirconia/ceria mixed oxides. xe2x80x9cZirconiaxe2x80x9d in the first place of xe2x80x9czirconia/ceriaxe2x80x9d indicates that zirconia is present in an amount which is at least equivalent but in general larger than the amount of ceria. Such a zirconia component may further be stabilized with the stabilizers mentioned above, namely yttria, neodymia, lanthana, praseodymia, gadolinium oxide or mixtures thereof at the expense of zirconia and ceria. Thus, the overall composition of the zirconia component may comprise of from 99.5 down to 45 wt.-% of zirconia and of from 0.5 to 55 wt.-% of ceria, yttria, neodymia, lanthana, praseodymia, gadolinium oxide or mixtures thereof, whereby zirconia is present in an amount which is equal to or larger than the amount of ceria.
The first support materials form the major part of the catalytic coating. The weight range of the first support materials relative to the second support material ranges between 1.1:1 to 20:1. The concentration of the first part of the total platinum amount of the catalyst on the first support materials (selected from active alumina, ceria/zirconia mixed oxide and the zirconia component or mixtures thereof) ranges between 0.01 and 5, preferably between 0.05 and 1 wt.-%, relative to the total weight of the catalyzed materials. Contrary to that, the concentration of platinum plus rhodium on the second support material (second active alumina) is preferably higher and lies between 0.5 and 20 wt.-% relative to the weight of the second support material with concentrations between 1 and 15 wt.-% being preferred. In total, platinum and rhodium together are present in the catalytic coating in concentrations of from 0.02 to 10 wt.-% relative to the total weight of the coating.
The catalyst carrier body used in the present invention is in the form of a honeycomb monolith with a plurality of substantially parallel passage ways extending therethrough. The passage ways are defined by walls onto which the catalytic coating is applied.
The passage ways of the carrier body serve as flow conduits for the exhaust gas of the internal combustion engine. When flowing through these passages the exhaust gas comes into close contact with the catalytic coating whereby the pollutants contained in the exhaust gas are converted into benign products. The carrier bodies may be manufactured from any suitable material, such as from metallic or ceramic materials as is well known in the art. The passage ways are arranged in a regular pattern over the cross section of the carrier bodies. The so-called cell density (passage ways per cross sectional area) may vary between 10 and 200 cmxe2x88x922. Other suitable carrier bodies may have an open cell foam structure. Metallic or ceramic foams may be used.
The catalytic coating is applied to the carrier body in amounts of from about 50 to 250 g/l. Advantageously the catalytic coating comprises of from 0 to 150 g/l, preferably of from 20 to 150 g/l, of said first active alumina and of from 10 to 100 g/l, preferably of from 20 to 100 g/l, of said ceria/zirconia mixed oxide component. The zirconia component may be present in concentrations of from 0 to 80 g/l, preferably of from 5 to 60 g/l.
For proper functioning of the catalyst it requires a sufficient oxygen storage capacity. The oxygen storage capacity of the catalyst is primarily supplied by said ceria rich ceria/zirconia component. In minor amounts also the zirconia component may provide a certain portion to the overall oxygen storage capacity of the catalyst. But in a preferred embodiment of the catalyst the oxygen storage capacity of the catalyst is solely based on ceria rich ceria/zirconia mixed oxide while the zirconia component is a pure zirconia material or zirconia stabilized with 0.5 to 10 wt.-% of the stabilizers already mentioned above.
The concentration of said second active alumina is preferably selected between 5 and 50 g/l. In a most preferred embodiment the first and second active alumina are the same and have a specific surface area between 50 and 200 m2/g and are stabilized with 0.5 to 25 wt.-% of lanthana, ceria, yttria, neodymia, gadolinium oxide or mixtures thereof. The oxygen storage component is advantageously selected from ceria rich ceria/zirconia mixed oxides containing 60 to 90 wt.-% of ceria and additionally stabilised with 0.5 to 10 wt.-% of praseodymia (Pr6O11).
For the purpose of suppressing the emission of hydrogen sulphide the catalytic coating may further comprise from about 1 to 30 g/l of a nickel, iron or manganese component.
The surface area of the support materials for the noble metal components is important for the final catalytic activity of the catalyst. Generally the surface area of these materials should lie above 10 m2/g. The surface area of these materials is also called specific surface area or BET surface area in the art. Preferably the surface area of the materials should be larger than 50 m2/g, most preferably larger than 100 m2/g. Active aluminas with a surface area of 140 m2/g are conventional. Oxygen storage components based on ceria or ceria/zirconia mixed oxides are available with surface areas of 80 m2/g up to 200 m2/g, depending on the state of calcination upon delivery. Besides this, there are also available so-called low surface area ceria materials with surface areas below 10 m2/g. Zirconia materials with 100 m2/g are also conventional.